Fiber-optic current sensors rely on the magneto-optic Faraday effect in an optical fiber that is coiled around the current conductor. The current-induced magnetic field generates a circular birefringence in the optical fiber that is proportional to the applied magnetic field. A preferred arrangement employs a reflector at the sensing fiber's far end so that the light coupled into the fiber performs a round trip in the fiber coil. Commonly, left and right circularly polarized light waves, which are generated from two orthogonal linearly polarized light waves by a fiber-optic phase retarder spliced to the sensing fiber and acting as quarter-wave retarder (QWR), are injected into the sensing fiber as described in the references [1-4]. After the round trip through the fiber coil the two circular waves have accumulated a phase delay proportional to the applied current as a result of the circular birefringence in the fiber. This phase delay is proportional to the number of fiber windings around the current conductor, the applied electrical current, and the Verdet constant V(T, λ) of the fiber. The Verdet constant is thereby material-, temperature-, and wavelength-dependent.
As an alternative, the sensor may be designed as a Sagnac-type interferometer with quarter-wave retarders (QWRs) at both sensing fiber ends and light waves of the same sense of circular polarization that are counter-propagating in the sensing fiber.
In references [3, 5, 6] methods are disclosed where the fiber retarder is employed to balance the temperature dependence of the Verdet constant, which is (1/V)dV/dT=0.7×10−4° C.−1 for fused silica fiber. For this purpose, the retardance of the QWR is set to an appropriately chosen value that commonly deviates from perfect quarter-wave retardance. The variation of the retardance with temperature changes the sensor scale factor such that it balances the variation of the Verdet constant with temperature. A method how to manufacture such retarders is disclosed in reference [7].
Typically, fiber-optic current sensors employ semiconductor light sources such as superluminescent diodes (SLDs). If the source is not temperature stabilized, its emission spectrum shifts to smaller wavelengths at increasing ambient temperature. Typical SLD wavelength shifts correspond to a few tenths of a nanometer per ° C. depending on details of the SLD material system. The Verdet constant for fused silica fiber roughly scales with wavelength as V(λ)=V(λ0)(λ0/λ)2, as stated for example in references [8, 9]. Here, λ0 is the initial wavelength (reference wavelength). The relative scale factor variation at 1310 nm is thus −0.15%/nm or −0.05%/° C., if one assumes a wavelength shift with temperature of 0.3 nm/° C. Apart from the Verdet constant, other sensor parameters may also change with source wavelength, for example the retardance of the fiber retarder at the beginning of the sensing fiber, and may thus influence the scale factor. The source wavelength also varies with drive current or may shift as a result of source aging.
To counter the shift in wavelength it is known to stabilize the wavelength of light source by operating the source at constant current and stabilizing the source temperature by means of a thermoelectric cooler (TEC) or by monitoring the source temperature and appropriately correcting the sensor signal. Wavelength shifts due to source aging are not corrected when using these methods.
In the field of fiber-optic gyroscopes, several methods have been reported to track wavelengths shifts in gyroscopes, including the use of a tracking interferometer [10], wavelength division multiplexers [11-13], or fiber gratings [12, 14].
Further known are voltage or electric field sensors based on the Pockels effect or linear electro-optic effect or on the use of an optical fiber coupled to a piezo-electric material. In these sensors, birefringence induced by the electric field or by force or anisotropic change in the refractive index of the material is used in the fiber optic sensor to measure voltages, electric field strength or force.
As discussed above, references [3, 5, 6] disclose how to compensate a temperature dependence of a fiber-optic current sensor. Hereby, generally, a first-order, i.e. linear, temperature-dependent contribution that e.g. originates from the temperature dependence of the Verdet constant (dV/[VdT]=0.7×10−4° C.−1 in fused silica fiber at 1310 nm) is counteracted by a temperature-dependent behaviour of a fiber retarder at the entrance to the sensing fiber. WO 2014/154299 further discloses how to compensate a temperature dependent signal that originates from a combination of the temperature dependence of the Verdet constant and the birefringence of the sensing fiber. However, these methods generally do not compensate a potential higher order, in particular second order, i.e. parabolic or quadratic, temperature dependence or can even introduce an additional higher order temperature dependence. Depending on the detection method of the sensor (detection with non-reciprocal phase modulation or detection with passive phase bias), the curvature of the parabolic contribution can either be positive or negative. A non-linear contribution can also originate from polarization cross-coupling due to packaging-related fiber stress at extreme temperatures or temperature-dependent shifts of the optimum sensor working point, as e.g. shown in WO 2014/006121. U.S. Pat. No. 5,696,858 discloses a fiber-optic sensor according to the precharacterizing clause of the independent claims.
It is an object of the invention to provide a fiber optic sensor, particularly a magnetic field sensor or current sensor that includes a sensing fiber to be exposed to a magnetic field, where the sensor signal is less sensitive to wavelength shifts of the light used and/or to temperature shifts.